Design And Analysis Of NMR Radio Frequency Power Amplifier

Magnetic resonance imaging(MRI) has advantage of high resolution, multi axis imaging, and no radioactivity. It gradually becomes an important direction of medical imaging. At present, the domestic MRI system mainly depends on import, so MRI system which has stable performance and inexpensive price plays an important role in improving the health level in our country.RF power amplifier is a key component of MRI system. With the increasing demand of MRI imaging, the performance of RF power amplifier has been put forward higher requirements constantly. Technology of power amplifier has been developed rapidly, however, foreign researchers are leading in both power devices and corresponding circuit design field. In the practical application about MRI system, how to improve the efficiency and the linearity with the RF power amplifier is still research focus.In this paper, according to the characteristic of RF power amplifier in 1.5T MRI system, we design and analysis a plan for wideband RF power amplifier. Through the control of peripheral control circuit and three stage power amplifier, the power of RF signal is amplified, and at the same time we design several key modules’ hardware and verify the feasibility though software. The main works are as follows:1. The first and second stage power amplifier amplifies the RF signal preliminary.The first stage power amplifier greatly facilitates the circuit design by using power amplifier chip. The second stage power amplifier is a solid-state power amplification circuit based on LDMOS MRF6V2150 N, and it has advantage of high power, high linearity, and though the use of the feedback structure to optimize the gain flatness and input and output VSWR. The second stage power amplifier is optimized by using ADS simulation, and the result shows that the output power exceeds 48 dBm in the working frequency band, and the third harmonic, power added efficiency and VSWR also exceed the requirements. Design of second stage power amplifier also includes its thermal design, the experimental result shows that the indicators including the heat plate and waveform distortion have reached the design requirements.2. Peripheral control circuit contains of a frequency detection circuit, RF switch and a voltage control attenuator, these modules work together to control the poweramplifier’s working state. Frequency detection circuit can measure the frequency of RF signal by PLL technology; RF switch which belongs to single pole double throw switch can control two second stage power amplifiers’ working state; Voltage control attenuator can adjust the power of RF signal by the control voltage to prevent RF input power is too large and to reduce the gain flatness.3. Because of the advantage of triode such as huge power output ability and stability,triode are still widely used, so the third stage power amplifier are using 3CX2500A3 to accomplish the final magnification of the RF signal. The PSpice model of triode is established by using the working curve, and we also verify the feasibility of the model.Finally, a power amplifier circuit based on triode is established, and the simulation is conducted by PSpice. The result shows that the output meets the requirements of the1 KW, and the output waveform is good, without significant distortion.